Analysis and modeling of coastal hazards to the desalination plants in the Baltic Sea – disentangling the impacts of a coastal storm in summer 2023

Abstract. We present an analysis of coastal hazards associated with the passage of an extreme storm named Hans in the Baltic Sea in August 2023. The storm resulted in disturbance of drinking water production at the desalination plants on Gotland, the largest island of the Baltic Sea. The limited ground water resources combined with increased demand during the warm tourist season lead to recurring seasonal water stress on Gotland. Thus, drinking water production through desalination of sea water is needed to complement the municipal water supply. The storm Hans triggered extreme water and organic material transport to the intake stations of the desalination plants clogging the filters, and coinciding with cold sea temperature spells, that collectively disturbed the water treatment process. We analyze the ocean-dynamical drivers of these coastal hazards and their impacts, and present a pathway toward a tailored forecast system to the desalination plants combining available observations, operational ocean model outputs, and statistical models. The linear coastal response to southwesterly winds of storm Hans was found to be the primary driver, either directly or indirectly, of the coastal hazards impacts. The linear regression models building upon this finding show a potential for future development of forecast framework to inform the water management on Gotland, if continued reporting and observational efforts can be secured.